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This PhD project aims to develop lead-free thin-film dielectric materials for fixed value, voltage tunable and high-k zipping variable capacitors using growth techniques that can be scaled for silicon batch fabrication. The thesis specifically details the growth and characterisation of barium zirconate titanate (BZT) and bismuth zinc niobate (BZN) dielectric thin films. Fixed value and tunable capacitors have been realised through the use of low and high permittivity dielectric thin film materials in both the amorphous and crystalline states. Planar devices fabricated using BZT and BZN thin-film dielectrics were grown by sol-gel and RF magnetron sputtering, respectively. The effects of different bottom electrodes were also investigated. Capacitors in metal-insulator-metal (MIM) structure have been fabricated to characterise the dielectric films at low frequency (to 300 kHz). Finding alternative higher permittivity dielectrics to SiO2 for various capacitor and isolation layer applications can be a challenge. Trials were conducted that looked at using amorphous BZT and nano-crystalline/crystalline BZN as a low-k dielectric insulator. Dielectric constants of ~50 were typical for BZN, but much lower permittivity was achieved for amorphous BZT, between 2 and 15 when deposited on Cr/Au bottom electrode. Breakdown strength of amorphous BZT was extremely high (2.0 MV/cm) and far superior to that of BZN (0.35 MV/cm). The dielectric strength of BZN was increased to 0.56 MV/cm when BZN was grown on a BZT seed layer due to a change in the microstructure of the BZN film from granular to columnar. The development of a suitable dielectric BZN for use with polymer substrate was also investigated and MIM capacitors fabricated by sputter deposition. Preliminary results for nano-crystalline BZN thin film growth on gold coated liquid crystal polymer (LCP) substrates appeared encouraging with dielectric constant of 27 and loss 0.005. Crystallised BZT thin films can be used to good effect as lead-free dielectric material in tunable devices. For BZT in the ferroelectric phase, excellent tunabilities of 80% were realised when deposited on platinised silicon. This wasfound not to be the case for BZT in the paraelectric phase where tunabilities tended to be ~60% at best. The dielectric properties of thin-film MIM capacitors can be enhanced by the use of lower resistivity bottom electrode such as gold. Previous research has failed to demonstrate growth and crystallisation of BZT on gold electrode due to the fact that it is technically difficult using the sol-gel method. Films tend to crack after annealing. I have found that films can be stabilised, and the tunability of BZT thin film in the paraelectric phase can be increased significantly, by growing BZT on gold bottom electrode using a BZN buffer layer 25nm thick. A peak tunability of 83% was achieved while maintaining a low dielectric loss of ~0.01. Novel BZT multilayer structures incorporating both ferroelectric and paraelectric compositions were grown on platinised silicon resulting in a tunability of 82% at a bias field 600 kV/cm. Based on the success of growing good quality BZN thin films on gold bottom electrode, it was decided to use BZN thin film as one of the high-k dielectrics in the zipping varactor, a miniature MEMS tunable device. Trials were performed that looked at depositing BZN on thick (800nm) gold coated silicon and glass. This was successful on small sample pieces but failed when scaled up to full device wafer size (100 mm diameter) due to Cr/Au diffusion into the dielectric layer. To overcome this, a 300nm thick BZN film was sputter deposited on Ti/Pt and Ti/Au/Pt coated 100 mm glass device wafers and processed to form the dielectric layer and bottom electrode of the capacitor. As part of the process the BZN layer required patterning. Wet etching of the small features was inappropriate due to undercutting of the structure; dry etching was therefore necessary. Prior to this work there had been nothing reported on the dry etching of BZN, only wet etched using a 1:10 HF-deionised H2O solution. In this work, thin-film BZN was reactively ion etched in Ar/CHF3 plasma at a rate of 6nm per minute with excellent selectivity over platinum of 10:1. Fabrication of the curved top electrode, final assembly and device testing were undertaken by a group at Imperial College London who were collaborators on a work programme entitled, “Integrated Functional Materials for System-in-Package Applications”.

Since its advent in the early 1980s, Scanning Tunnelling Microscopy (STM) has been used to advance the knowledge of semiconductor grow processes. Hybridisation of STM with other analytical methods and the Molecular Beam Epitaxy (MBE) growth technique allowed a flexible and diverse approach to growth front exploration. The first hybrid, limited the applicability of STM to in vacuo operation whereby the sample is rapidly cooled or 'quenched” in an attempt to preserve the growing surface, before imaging can commence. This technique suffers dually from the unknown effects of the quenching procedure and the limiting ability to only capture frozen-in-time images of the surface. The ultimate evolution of STM would be to allow concurrent or in situ MBE and STM operation. The ability to perform concurrent MBE and STM requires three basic criteria: accurate and stable control of the sample temperature, reliable and maintainable STM tunnelling tip procedures and controlled, sustained emission from the MBE effusion cells within the STM chamber. Samples are slivers 8 x 1 mm2 to 12 x 4 mm2 of wafer mounted for either direct current heating or radiative pyrolytic boron nitride heating within the STM chamber. No direct temperature monitoring method is available and thus a myriad of techniques were employed to map the current-temperature response for samples including Reflection High Energy Electron Diffraction (RHEED), thermocouples and thermography, yielding a reliable heating profile. Tunnelling tip fabrication involves manufacturing an atomically sharp tip via a two-step electrochemical etching and annealing procedure. An extensive and exhaustive investigation sought to produce a quantitative method for tip identification and etching parameterisation based on the available variables of differential sensitivity, etching voltage, immersion depth and etchant concentration. An optimised tip type transfer diagram of tip fabrication resulted, after which, an anneal algorithm was formulated resulting in clean, sharp tips without the side effect of apex distortion and melting. Quality of the initial growth layer depends strongly on the clean-up conditions. As a prequel to growth, sample preparation methods are investigated via STM analysis to determine the best preparation conditions in order to achieve high quality MBE growth in the STM chamber. The final stage involves MBE source operation during STM. Initial investigation focused on flux alteration of surface reconstructions and allowed the effects of As4 on the STM stage to be investigated. This is the first documented case where an e-beam As4 source has been successfully operated within an STM system, during imaging. The inclusion of group III elements in the evaporation flux proves unequivocally that III-V Molecular Beam Scanning Tunnelling Microscopy (MBSTM) is a realisable investigatory technique. Simultaneous deposition of In and As whilst imaging allowed dynamic observation of the InAs/GaAs wetting layer evolution on GaAs(001)-(2 × 4). The experiment followed initial heteroepitaxial growth through wetting layer evolution to the onset of 3D growth.

Path integral quantum Monte Carlo (PI-QMC) is a powerful technique, which can be used to model the properties of multiple interacting particles at finite temperatures. In this work path integral quantum Monte Carlo has been applied to the problem of few particle interactions in quantum dots and other semiconductor nanostructures. Quantum dots are currently the subject of much research and in order to further understand their properties it is necessary to perform theoretical modelling. In this work, the method by which the problem of the attractive Coulomb potential was overcome is detailed. Following that, comparisons are made between . experimental data and PI-QMC results for excitonic complexes in 111-V dots. Both the energies and voltage extents were found to show good agreement between experiment and theory. Comparisons are also between theory and experiment of II-VI, with experimental data using a harmonic potential to model the dot. Again, good agreement is seen. Finally, as an example of the power of PI-QMC, the behaviour of electrons and holes is modelled for alternative nanostructures, such as coupled quantum dots, quantum rings and core-shell structures. With some simple modifications, the same PI-QMC method could be used.

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In 1965 Gordon E. Moore forecasted that the number of transistors on a chip would double every 18 months. This became known throughout the semiconductor industry as Moore's law. However, it was observed that this cannot be continued indefinitely but, it has been seen as a goal for the industry. Since the 1960s, semiconductor manufacturers have strived to scale down device features according to Moore's law. This has been achieved by photolithography, and has unquestionably been one of the major driving forces behind the progress made in the field of semiconductor technology. It is in this miniaturization of features that photolithography has made its contribution, and where the advancement of the fabrication techniques to produce a mask has played its role. In this thesis the focus was on investigating suitable technologies to economically maintain the trend of Moore's Law. The fabrication of next generation optical photomasks was studied that could produce the next generation ultra large scale integration (ULSI). Two methods were developed and contrasted these were both based on 'top down' photolithography using I-line radiation. However, two different approaches were adopted to fabricate these optical masks. The first was based on electron beam lithography by a fabricating a novel nanocomposite electron beam resist that incorporated nanoparticles into Polymethylmethacrylate (PMMA). The nanoparticles were used to attenuate the radiation propagating through the electron beam resist as the PMMA was found to be transparent at the wavelength of the incident radiation. When the nanocomposite resist was patterned by the electron beam, the pattern was transferred to a photoresist via contact printing using the conventional photolithography technique. The second approach exploited a novel photolithography technique using periodic hexagonally closely packed silver nanoparticle 2D arrays. A method to precisely control the spacing between nanoparticles by temperature has been demonstrated this was then used to transfer a nano - pattern into a photoresist. The high - density nanoparticle thin film was accomplished by self-assembling through the Langmuir - Schaefer (LS) technique on a water surface and transferring the nanoparticle monolayer to a temperature sensitive polymer membrane. A 30nm hexagonally packed silver nanoparticle 2D array pattern with a 50nm period has been successfully transferred into photoresist. The resultant feature sizes were 34nm with a period of 16nm, due to the surface plasmon resonance where the photoresist is approximately 11 times smaller than wavelength. This work demonstrated the suitability of these novel masks in certain applications where the complexity of fabrications and the associated costs varied considerably. 'Top down' techniques were often expensive and slow, but provided a direct route to achieving the desired features. The 'bottom up' techniques were often achieved at low cost but were limited in terms of control over geometry.

The high temperature, wet oxidation of buried, high aluminium content AlGaAs layers has seen a lot of attention in recent years thanks to its ability to form good quality, deep and uniform oxide layers a unique attribute among the III-V materials. It is the unusual combination of relatively high density and good porosity of the oxide material, that allows the oxidation to reach much greater depths than that of other materials, and this is what makes the process so widely employable and versatile. This project is based on two systems of oxidisable layers, incorporated into two wafers, both of which were designed as part of the work. One wafer, referred to as Wafer A, contains 30nm 98% aluminium layers in both of the cladding regions, and the second, Wafer B, has spaced stacks of 1nm 95% aluminium layers placed directly into the active region. The oxidation depths of both wafers are mapped over a range of temperatures and times, and maximum measured rates of 2um/min and 1um/min are reported for Wafers A and B respectively. Further to this, a non-trivial dependence of the oxidation rate on dopant species is reported. It is found that although p-type material initially achieves greater oxidation rates, after a depth of approximately 8pm has been reached, the n-type material over takes. This result is contrary to the popular belief that p-type material oxidises faster, regardless of depth. The two wafers are then made into working devices. In the case of Wafer A, longitudinal oxidation of broad-area devices are used to create unpumped saturable absorber sections. These sections are responsible for various effects including the lowering of laser threshold currents by up to 30%, the narrowing of laser near-fields by up to 50%, and the production of repetitive wavelength shifting behaviour, which is investigated in great detail. Wafer B is oxidised laterally to produce current confinement directly in the active region. Threshold current reductions of around 20% are recorded. Lastly, cross-sectional electron microscope images reveal a simple and versatile method of fabricating buried microlenses from the structure of Wafer B.